AJP - Endocrinology and Metabolism, Vol 241, Issue 3 261-E267, Copyright © 1981 by American Physiological Society
Evidence for a photoperiod-sensitive pacemaker for estrous cycle of the rat
J. M. Yochim and H. W. Shirer
To define how the periodicity of light regulates reproductive cyclicity in
the rat, animals were exposed to slightly altered photoperiods in which the
light/dark ratio was maintained as in the control (14L + 10D) environment.
Exposure to a 22.5-h photoperiod (13.125L + 9.375D) for 10-80 days induced
a significant prolongation of the first and all subsequent vaginal cycles
by about 30 h. Ovulation in these animals was normal. By contrast, exposure
to a 26-h photoperiod (15.167L + 10.833D) had no significant effect on
vaginal cyclicity. A comparison of the data with theoreticaly generated
results revealed that a significant prolongation of the estrous cycle under
conditions of a shortened photoperiod cannot be accounted for by a simple
entrainment of the estrous cycle to the timing of the new circadian rhythm.
Rather, the evidence suggested that a photoperiod-entrained rhythm may
interact with a second rhythm of different periodicity to alter the
duration of the reproductive cycle as measured in this study.
